RE: [PATCH v2 1/1] usb: gadget: f_fs: Add support for SuperSpeed Mode

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> From: linux-usb-owner@xxxxxxxxxxxxxxx [mailto:linux-usb-owner@xxxxxxxxxxxxxxx] On Behalf Of Manu Gautam
> Sent: Thursday, September 26, 2013 12:08 AM
> 
> On 9/26/2013 2:10 AM, Felipe Balbi wrote:
> >
> > On Tue, Sep 24, 2013 at 03:00:20PM +0530, Manu Gautam wrote:
> >> Hi Felipe,
> >>
> >> I wanted to mention one point with respect to this patch: Below
> >> changes in the functionfs.h to add ss_count (super speed descriptors
> >> count) in desc_header (which is passed from userspace) make the driver
> >> incompatible with existing userspace applications compiled against old
> >> header file. Let me know if that is acceptable.  We are using this
> >> driver with Android for adbd (android debug bridge) and these changes
> >> are required to support adb over Super Speed controllers e.g. DWC3
> >> along with changed in adbd to pass SS EP and companion descriptors.
> >
> > Good you mentioned, it saves me the trouble of reviewing this patch :-)
> >
> > It's not acceptable to break userspace ABI at all. If you want
> > SuperSpeed support on function fs, we need to figure out a way to do so
> > without breaking userspace.
> >
> > This might mean adding a separate userspace interface to be used with
> > superspeed. While at that, we might want to add a few bytes of reserved,
> > unused space in our structures for situations where we need to add more
> > data into it, just to make it slightly future proof.
> >
> 
> Thanks for your reply.
> As you suggested we can have a different interface for super speed
> which would be optional to workaround ABI compatibility issue.
> Let me know if below interface looks fine to you, I will then implement
> accordingly:

Just a suggestion: Instead of a new interface for SuperSpeed, why not
just add the new fields to the end of the ffs_data struct? And have the
functions that copy the struct to/from userspace check the 'len' value
passed in, and only handle the SuperSpeed stuff if the length indicates
it is new userspace?

-- 
Paul

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